Voltage converters have been widely used for supplying regulated voltages for electronic devices. As shown in FIG. 1, a conventional boost converter 100 has an inductor L connected between a power input VIN and a phase node 106, a transistor 104 as a power switch connected between the phase node 106 and ground GND, an input capacitor CIN connected between the power input VIN and ground GND, a rectifier diode ZD connected between the phase node 106 and a power output VOUT, an output capacitor COUT connected between the power output and ground GND, and a boost control circuit 102 for switching the transistor 104 by a switching signal 103 to convert the input voltage VIN to an output voltage VOUT for supplying for a load RL. However, most of current electronic devices are provided with a protection function of under voltage lockout (UVLO), which will automatically shut down the electronic device if its input voltage is detected to be lower than a predetermined value, and therefore, at the moment the converter 100 is connected to a power source VIN, the output voltage VOUT of the converter 100 may have an initial value so low to cause the input voltage VIN to suffer a severe voltage sag, and thereby to further cause the other electronic devices that share the same power source VIN automatically shut down by the UVLO function. To prevent this accident, a protection function is further provided for the converter 100, which controls the converter 100 to enter into a pre-charge stage when it starts up, for the output capacitor COUT to be charged in advance to a predetermined value, for example VIN.
However, if the converter 100 is turned on after it has been shut down for a time period, or it is hot plug to a power source VIN, the input current Iin will suffer a current inrush due to the sudden charging to the input capacitor CIN and the output capacitor COUT, and the input current inrush may be so large to cause the input voltage VIN to suffer a severe voltage sag, and thereby to further cause the other electronic devices that are also connected to the power source VIN to automatically shut down by the UVLO function. To improve thereto, a conventional boost converter 200 as shown in FIG. 2 is proposed, in which a transistor 208 is connected between the phase node 106 and the output capacitor COUT, a boost control circuit 202 provides two switching signals 103 and 203 to alternatively switch the transistors 104 and 208, respectively, to convert the input voltage VIN to an output voltage VOUT, and a current limiting control circuit 210 is connected to the boost control circuit 202 and the transistor 208. When the converter 200 is hot plug to a power source VIN, it will enter into a pre-charge stage during which the current limiting control circuit 210 provides a control signal 213 to the gate of the transistor 208 to limit the maximum value of the input current Iin that charges the output capacitor COUT. In normal operation, the control signal 213 becomes a switching signal determined by the switching signal 203. However, in this scheme, the maximum value of the input current Iin that can flow through the transistor 208 in a pre-charge stage is constant, and it also limits the maximum value of the load RL accordingly. If the maximum value of the input current Iin in a pre-charge stage is selected higher such that the converter 200 can support a higher maximum loading, then the suppression on the input current inrush will become poorer.
Hence, it is desired an apparatus and method for suppressing the input current inrush for a voltage converter in a pre-charge stage, with capability of pre-charging any load.